Glossary
MC9S12UF32 Card Reader Reference Design User Manual, Rev. 0.1
40
Freescale Semiconductor
exception
— An event such as an interrupt or a reset that stops the sequential execution of the
instructions in the main program.
fetch
— To copy data from a memory location into the accumulator.
firmware
— Instructions and data programmed into nonvolatile memory.
free-running counter
— A device that counts from zero to a predetermined number, then rolls over to
zero and begins counting again.
full-duplex transmission
— Communication on a channel in which data can be sent and received
simultaneously.
hexadecimal
— Base 16 numbering system that uses the digits 0 through 9 and the letters A through F.
high byte
— The most significant eight bits of a word.
illegal address
— An address not within the memory map
illegal opcode
— A nonexistent opcode.
index registers (IX and IY)
— Two 16-bit registers in the CPU. In the indexed addressing modes, the
CPU uses the contents of IX or IY to determine the effective address of the operand. IX and IY
can also serve as a temporary data storage locations.
input/output (I/O)
— Input/output interfaces between a computer system and the external world. A CPU
reads an input to sense the level of an external signal and writes to an output to change the level
on an external signal.
instructions
— Operations that a CPU can perform. Instructions are expressed by programmers as
assembly language mnemonics. A CPU interprets an opcode and its associated operand(s) and
instruction.
inter-IC bus (I
2
C) — A
two-wire, bidirectional serial bus that provides a simple, efficient method of data
exchange between devices.
interrupt
— A temporary break in the sequential execution of a program to respond to signals from
peripheral devices by executing a subroutine.
interrupt request
— A signal from a peripheral to the CPU intended to cause the CPU to execute a
subroutine.
I/O
— See “input/output (I/0).”
jitter
— Short-term signal instability.
latch
— A circuit that retains the voltage level (logic 1 or logic 0) written to it for as long as power is applied
to the circuit.
latency
— The time lag between instruction completion and data movement.
least significant bit (LSB)
— The rightmost digit of a binary number.
logic 1
— A voltage level approximately equal to the input power voltage (V
DD
).
